  We report observations of CO(J=2-1) and CO(J=3-2) line emission towards the
quadruply-lensed quasar RXS J1131-1231 at z = 0.654 obtained using the Plateau
de Bure Interferometer (PdBI) and the Combined Array for Research in
Millimeter-wave Astronomy (CARMA). Our lens modeling shows that the asymmetry
in the double-horned CO(J = 2-1) line profile is mainly a result of
differential lensing, where the magnification factor varies from ~3 to ~9
across different kinematic components. The intrinsically symmetric line profile
and a smooth source-plane velocity gradient suggest that the host galaxy is an
extended rotating disk, with a CO size of R_CO~6 kpc and a dynamical mass of
M_dyn~8x10^10 Msun. We also find a secondary CO-emitting source near RXS
J1131-1231 whose location is consistent with the optically-faint companion
reported in previous studies. The lensing-corrected molecular gas masses are
M_gas = (1.4+/-0.3)x10^10 Msun and (2.0+/-0.1)x10^9 Msun for RXS J1131-1231 and
the companion, respectively. We find a lensing-corrected stellar mass of M_* =
(3+/-1)x10^10 Msun and a star formation rate of SFR_FIR = (120+/-63) Msun yr^-1
, corresponding to a specific SFR and star formation efficiency comparable to
z~1 disk galaxies not hosting quasars. The implied gas mass fraction of
~18+/-4% is consistent with the previously-observed cosmic decline since z~2.
We thus find no evidence for quenching of star formation in RXS J1131-1231.
This agrees with our finding of an elevated MBH/Mbulge ratio of >0.27+0.11%
-0.08 compared to the local value, suggesting that the bulk of its black hole
mass is largely in place while its stellar bulge is still assembling.
