Merge java-overridable/ into java/ again

This CL a logical revert of a previous CL [1], which separated Java
files into common ones and overridable ones.  Now that that overriding
concept is no longer used, there is no need to keep having separate
directory structure.

 [1]: Ic734bd4d20aa050c688a3158b1a382ae0ac18991
      fb74ab15

Fix: 79323502
Test: make -j aosp_taimen-userdebug && make -j
Test: tapas LatinIME && make -j
Change-Id: I2090bc25d18e6d4f24e91c2cbfe832755cbb4e8f