Cleanup in EmulatorClipboardMonitor (3)

make more functions static, less places to
mess with the global state.

Bug: 231345789
Test: presubmit
Signed-off-by: Roman Kiryanov <rkir@google.com>
Change-Id: I236b039c4fbbbfb519ba1432326bba9a89d008c2
Merged-In: I236b039c4fbbbfb519ba1432326bba9a89d008c2