Cleanup: fix return type of PPS::getMixedNaluTypesInPicFlag

Return bool since variable is a boolean. Also fix use of returned
values.
6 jobs for !2011 with clean51 in 13 minutes and 48 seconds (queued for 4 seconds)
detached
Status Job ID Name Coverage
  Build
passed #20367
macos
build_macos

00:04:41

passed #20368
ubuntu1604
build_ubuntu1604

00:05:23

passed #20369
ubuntu1804
build_ubuntu1804

00:05:13

passed #20370
ubuntu1804-gcc8
build_ubuntu1804-gcc8

00:05:41

passed #20371
vc191x
build_vc191x

00:13:47

passed #20372
vc192x
build_vc192x

00:13:48