[Partial Screenshare] Enable feature by default

Define a new flag name with default value of true in the platform
for the public launch. If we ever need to disable the feature, we
will create a new server-side flag matching this name.

Released flag provides an easy way to read the server-side flag.
WM_ENABLE_PARTIAL_SCREEN_SHARING  is relied upon throughout SysUI to
check if Partial Screenshare is enabled.

Bug: 285183655
Bug: 254513207
Change-Id: I41d8da4ec0b602327db9c3c2fac05722b5831f38
Test: Bugbash last week & manually overrode to off through adb, QA test in b/269459647